Molecular phylogenetic data and seed coat anatomy resolve the generic position of some critical Chenopodioideae (Chenopodiaceae - Amaranthaceae) with reduced perianth segments

Abstract

The former Chenopodium subgen. Blitum and the genus Monolepis (Chenopodioideae) are characterised in part by a reduced (0-4) number of perianth segments. According to recent molecular phylogenetic studies, these groups belong to the reinstated genera Blitum incl. Monolepis (tribe Anserineae) and Oxybasis (tribe Chenopodieae). However, key taxa such as C. antarcticum, C. exsuccum, C. litwinowii, C. foliosum subsp. montanum and Monolepis spathulata were not included and so their phylogenetic position within the Chenopodioideae remained equivocal. These species and additional samples of Blitum asiaticum and B. nuttallianum were incorporated into an expanded phylogenetic study based on nrDNA (ITS region) and cpDNA (trnL-trnF and atpB-rbcL intergenic spacers and rbcL gene). Our analyses confirm the placement of C. exsuccum, C. litwinowii and C. foliosum subsp. montanum within Blitum (currently recognised as Blitum petiolare, B. litwinowii and B. virgatum subsp. montanum, respectively); additionally, C. antarcticum, currently known as Oxybasis antarctica, is also placed within Blitum (reinstated here as B. antarcticum). Congruent with previous studies, two of the three accepted species of Monolepis - the type species M. trifida (= M. nuttalliana) as well as M. asiatica - are included in Blitum. The monotypic genus Carocarpidium described recently with the type C. californicum is not accepted as it is placed within Blitum (reinstated here as B. californicum). To date, few reliable morphological characters have been proposed that consistently distinguish Blitum (incl. two Monolepis species) from morphologically similar Oxybasis; however, two key differences are evident: (1) the presence of long-petiolate rosulate leaves in Blitum vs. their absence in Oxybasis and (2) a seed coat structure with the outer wall of the testa cells lacking stalactites ('non-stalactite seed coat') but with an obvious protoplast in Blitum vs. seed coat with the outer walls of the testa cells having stalactites ('stalactite seed coat') and a reduced protoplast in Oxybasis. Surprisingly, the newly sequenced North American Monolepis spathulata nested within the tribe Dysphanieae (based on ITS and trnL-trnF + rbcL + atpB-rbcL analyses).The phylogenetic results, as well as presence of the stalactites in the outer cell walls of the testa and lack of the rosulate leaves, confirm the distinctive nature of Monolepis spathulata from all Blitum and, therefore, the recent combination Blitum spathulatum cannot be accepted. Indeed, the morphological and molecular distinctive nature of this species from all Dysphanieae supports its recognition as a new monotypic genus, named herein as Neomonolepis (type species: N. spathulata). The basionym name Monolepis spathulata is also lectotypified on a specimen currently lodged at GH. Finally, while Micromonolepis pusilla is confirmed as belonging to the tribe Chenopodieae, its position is not fully resolved. As this monotypic genus is morphologically divergent from Chenopodium, it is retained as distinct but it is acknowledged that further work is required to confirm its status.